The part that caught me be surprise is when the interpreter finds such code, it drops through to code that is two or three years old. This new effort, JaegerMonkey, looks to fix that, shoring up this performance drag on more complex scripts. It will marry the approach, compiling entire functions to assembly, proven out by the other modern browsers with TraceMonkey on the promise that a hybrid approach will yield the benefits of both.
There is plenty of more detail at the article, including some possible concerns arising from too much parasitizing of approaches between the competitive browsers. While there is no projected date for when JaegerMonkey could be included in a generally available build, the developers look to be doing the key integration work necessary for that this week.